Electronic circuitry continues to grow in complexity while simultaneously shrinking in size and cost. The resulting circuit density increase has placed large demands on production throughput of high-density integrated circuits, hybrid circuits, and ECBs.
Prior workers have employed ganged mechanical drills and punches to process holes in ECBs, but the diameters of the holes are larger than new hole diameter requirements dictate. Moreover, mechanical drilling methods have been slow, prone to tool breakage, and limited to drilling so-called “through” holes.
More recently, laser-based drilling methods have evolved that enable processing each second hundreds of very small holes (referred to as “microvias” or “vias”) that often terminate on conductor layers within the ECBs.
For some drilling applications, a Gaussian distributed laser beam is used to process the material, and this beam has a diameter significantly smaller than the diameters of the holes being drilled. Therefore, the laser beam must be moved to either excise the hole or ablate its entire area. The types of motion and constraints on the motion directly impact the time taken to drill a hole, and hence the laser system throughput.
Prior workers have laser-drilled holes with so-called “trepan” and “spiral” motion patterns, which are commonly referred to as “tools.” Trepan processing starts at the center of the hole, then moves rapidly to the hole perimeter and spins the beam for a programmed number of repetitions around the perimeter before returning rapidly to the center. Spiral processing starts at the center of the hole, moves rapidly to an inner diameter, then spins the beam positioner for a programmed number of revolutions, incrementing the diameter until the hole perimeter is reached. Laser beam movements may be carried out by a wide variety of laser beam positioning systems, such as the Model 53XX series of workpiece processing systems, manufactured by Electro Scientific Instruments, Inc., of Portland, Oreg., the assignee of this patent application.
Prior trepan and spiral laser hole drilling methods present at least the nine problems set forth below:
1. Prior tool patterns cause undue acceleration limits on positioner systems. Prior art trepanning entails moving the laser beam in a circular motion around the perimeter of the hole being processed. Skilled workers know that the radial acceleration of circular motion is equal to ν2/R, where ν is the tool velocity, and R is the radius of the circular motion. After positioning the tool to the center of the hole, trepanning is preceded by an initial move segment that transitions in a smooth manner between the center of the hole and the start of circular motion to limit the tool acceleration and jerk (rate-of-change of acceleration). With prior art trepanning, the acceleration required by the initial move segment is 2ν2/R, which is twice the acceleration required for the circular motion. Moreover, the motion axis requiring the double acceleration is the same axis executing a half-time duration acceleration pulse, resulting in a jerk profile four-times greater than circular motion requires. The laser beam positioning system acceleration is limited because twice the motor current is demanded at twice the servo frequency.
2. Prior spiral tool patterns are limited to outward spiraling, which limits the types of material that can be processed.
3. Prior trepan and spiral tools require time-wasting multiple steps for processing a hole with both spiral and repeated perimeter motions. Executing multiple steps requires the beam positioner to perform a generic move algorithm that requires at least two acceleration pulses to move the tool back to the center of the hole between steps.
4. If beam positioner settling time is required for recovering from a high acceleration, high velocity move from a prior hole, the settling time is implemented by a constant tool velocity move to the next hole target location, which limits the available beam positioner motion range. This motion range is significant when employing galvanometer-based beam positioners.
5. The above-described settling time technique also fails to settle the beam positioner at the steady state hole processing frequency, which causes a transient motion response when oscillatory circular motion begins.
6. Prior tool patterns are unduly slow when multiple repetitions of a spiral tool are required to approach the hole from various entry angles. Prior beam positioner methods employ the above-described generic move algorithm that requires at least two acceleration pulses to return to the center of the hole between repetitions.
7. Prior trepan tool patterns may cause uneven removal of material. This is so because the laser beam energy is concentrated in one quadrant of the hole as the beam moves from the center of the hole to the perimeter, and back again.
8. Prior spiral and trepan tools do not synchronize the timing of laser triggering signals with beam positioner motion, which causes an omission of a first hole processing pulse because typical Q-switched lasers do not generate a first pulse on command.
9. Prior trepan tool patterns used for drilling holes with multiple repetitions at the perimeter, substantially overlap laser pulses around the hole perimeter, and thereby cause uneven removal of material.
What is still needed, therefore, are lower cost, higher throughput workpiece processing machines having tool patterns that produce smaller, high-quality holes in a variety of workpiece materials, such as virtually any printed wiring board material, whether rigid or flexible, copper-clad or exposed, fiber reinforced, or homogeneous resin dielectric. The workpiece materials may also include ceramic substrates and silicon substrates, such as those employed in semiconductor devices.